Information for the control of strangles in horses
Note Number: AG1363
Published: August 2008
Updated: July 2018
This document provides practical information on how to manage and control strangles in horses.
Strangles is a notifiable disease in Victoria under the Livestock Disease Control Act 1994, which provides for the monitoring and control of livestock diseases in Victoria. All suspected or confirmed cases of strangles must be reported to Victoria’s Chief Veterinary Officer within seven days.
What is strangles?
Strangles is a highly contagious disease of the upper respiratory tract. It is caused by a bacterium called Streptococcus equi subspecies equi (S. equi) and affects horses, donkeys and mules.
Outbreaks of strangles may occur when different groups of horses mix together (e.g. at an equestrian event) or after the introduction of a new horse(s) onto a property. It is usually more common in younger horses; however, horses of any age can become infected.
Clinical signs of strangles (usually seen within three to eight days of a horse being exposed) can include:
- the rapid onset of pyrexia (high temperature); 39.5 to 41.5oC)
- a loss of appetite
- yellowish discharge from the nostrils
- enlarged glands in the head and neck, that often form abscesses
- coughing, and
- difficulty swallowing (hence the term ‘strangles’).
Clinical signs can last for days to months. Abscesses usually rupture and drain within two weeks. When abscesses burst, thick yellow pus is discharged, and recovery is generally without incident.
The incubation period of the disease is usually about one week, but may be up to three weeks.
Transmission occurs through both oral and nasal routes. It can occur via direct contact between individual horses and/or through indirect contact. Contaminated feed, water, bedding, stables, stable utensils and transport vehicles are important in the spread of infection. Good biosecurity is essential to prevent the spread of strangles between horses.
S. equi can survive for long periods in the environment, surviving in purulent discharges (pus) for months and in nasal discharges for several weeks.
Bacterial shedding usually ends rapidly after clinical recovery and can be confirmed by negative culture of nasopharyngeal swabs (swabs from the back of the throat). However, shedding may be intermittent and some horses become long-term carriers. Therefore, before any convalescent horse (or any in-contact horse) can be considered likely to be free of infection, a series of negative swabs is required.
A veterinarian will be able to confirm a strangles diagnosis by taking a swab from the back of the nasal cavity. Definitive diagnosis is based on PCR testing and often culture of S. equi. Culture may fail to isolate S. equi if antibiotic treatment has already commenced, the bacteria are present in low numbers or overgrown by other species. PCR testing is more rapid and generally superior in terms of sensitivity in these instances.
A rapid serological test is available at the Centre for Equine Infectious Diseases at The University of Melbourne Veterinary Biosciences Department. A series of two blood samples (serum) can screen for antibodies, indicating recent infection or exposure to S. equi. Samples should be spaced 14 days apart to allow for detection of antibodies arising from recent infection.
Immediate veterinary advice should be sought to alleviate the symptoms in individual horses, and to prevent its spread to other horses. Each case should be assessed and treated individually by a veterinarian, as clinical signs and the severity of the disease can vary between horses.
Affected horses should be isolated for six to eight weeks to prevent spread to other horses. Good biosecurity must also be observed when handling infected horses.
In cases where the disease is not severe, the best treatment is supportive therapy. Veterinary advice should be sought. Anti-inflammatory treatment may be necessary if the horse is suffering from painful swellings and/or a high temperature.
Early treatment with antibiotics may be given in some cases, however they are not routinely required for treatment unless clinical signs warrant their use. Antibiotics are not always indicated as they cannot penetrate the centre of an abscess where there is no blood supply. If required, penicillin type drugs are the antibiotic of choice against these streptococci.
If antibiotics are administered prior to sampling the horse, negative test results may be difficult to interpret.
Treating abscesses once they have burst can be done by flushing the site with warm saline or antiseptic.
While most horses recover with supportive treatment, complications may occur in up to 20% of affected horses. Complications include:
- bastard strangles – bacteria may spread through the body and can cause abscesses in the lungs, liver, spleen, kidneys, brain, and lymph nodes. These cases can be fatal.
- purpura haemorrhagica – bleeding from smaller blood vessels can cause red lesions/spots to form on the skin and mucous membranes. It may also present as swelling of the limbs and head. This condition is generally fatal.
- chronic carrier state – a small number of horses carry the strangles-causing bacteria in the guttural pouch (an out-pouching of the back of the throat) for months after they have recovered from clinical disease. Carrier horses appear healthy but shed bacteria in nasal discharges and are a source of infection for other susceptible horses.
Controlling the spread of strangles
The spread of infection can be controlled through the isolation of infected horses until they are free from infection. Spread can be limited by the early detection of shedders amongst newly-affected (and in-contact) horses by taking three nasopharyngeal swabs over a two-week period and culturing the swabs for S. equi. Three negative swabs provide strong evidence of freedom from infection in most cases.
All infected (and in-contact) horses should be placed under veterinary supervision in strict isolation with the highest possible biosecurity. When strict isolation fails to prevent the spread of infection, this is usually due to a breakdown in biosecurity. Horses should not enter an affected property unless they are vaccinated and can be kept in strict isolation from all sources of infection.
No infected or in-contact animal should be released from isolation until three consecutive negative swabs have been taken over a two-week period. If animals are found to be infected with S. equi for more than two months, an investigation should be conducted by a veterinary practitioner to identify and treat the site of the infection (e.g. the guttural pouch).
Vaccination against strangles provides good protection to horses. However, vaccination alone should not be considered an absolute preventative due to differences between individual horses, the time since vaccination, and the level of challenge to immunity.
Vaccination will assist control the spread of disease during an outbreak but horses already showing signs of infection should not be vaccinated.
Horses that attend pony club, shows or other equine events (including racing), or those that go to stud, are at particular risk and a regular vaccination program for strangles is strongly recommended.
An initial course of three injections two weeks apart is necessary. Booster vaccinations are recommended every six months as the duration of immunity following vaccination is short.
In addition to vaccination, all newly introduced horses should be monitored closely for three weeks after arrival, and ideally quarantined from other horses during that period. Any horse that develops signs consistent with strangles should be isolated immediately and examined by a veterinarian to exclude the possibility of strangles.
Vaccination is the best protection for a horse against strangles, but it is not completely effective due to differences between individual horses and time from vaccination. Vaccination will assist control by reducing the severity and duration of clinical disease and the spread of disease in an outbreak. An initial course of 3 injections two weeks apart is necessary. Booster vaccinations are recommended every 6 months as the duration of immunity following vaccination is short.
Horses should not be vaccinated while actively infected.
Strangles is a notifiable disease
Under the Livestock Disease Control Act 1994, a person must report strangles if they know or have reason to suspect that strangles is present in horses or horse products. This includes if the horse is:
- owned by that person or in the possession, control or charge of that person;
- on land owned and occupied by that person; or
- dealt with by that person as a veterinary practitioner, an inspector under the Meat Industry Act 1993 or the Export Control Act 1982, operator of a meat processing facility licenses under the Meat Industry Act 1993 where a quality assurance program is in force, the owner or person in charge of premises registered as a veterinary diagnostic laboratory, knacker, stock agent or other person dealing with livestock products or hives by way of a profession, trade or business.
The reporting of diseases such as strangles provides assurances to countries where horses are exported and where international equine events are held. Many of these countries require certification that the horse’s property of origin has been free from strangles for a specific period prior to export.
Reporting cases of strangles also provides assurances that Victoria is regularly monitoring for emergency diseases that are exotic to Australia (but have a similar presentation to strangles). The early detection of diseases such as equine influenza is imperative for the control of their spread and the protection of Australia’s horse population.
Properties are not placed under quarantine (or any other regulatory restrictions) when they report strangles.
Recommendations may be provided to assist control further spread of the disease.
Strangles can be reported to local animal health staff at Agriculture Victoria or by completing the notifiable diseases report form.
All reports of strangles to Agriculture Victoria are treated confidentially, unless the horse owner has authorised the release of the information. Information about the disease status of a property or livestock can only be released if the Secretary, Agriculture Victoria, determines that the release of information is in the public interest, for example, if public health or international trade is compromised.
Further information can be found at:
Agricultural Note AG0753 - Diseases of Horses Notifiable in Victoria for full list of Diseases
Agricultural Note AG1285 - Health and Biosecurity Guidelines for Transport of Horses
Agricultural Note AG1360 - Equine Biosecurity and Best Health Practice – For Equine Owners
Agricultural Note AG1361 - Equine Biosecurity and Best Health Practice – For Holding Equestrian Activities
Agricultural Note AG1362 - Equine Biosecurity and Best Health Practice – For Equine Service Providers
This Information Note was originally developed by Dr C El- Hage – June 2006.
This information note was revised and updated by S Forrest, Chief Veterinary Officer’s Unit in August 2008; by Dr S Tate, Chief Veterinary Officers Unit with the assistance of Equine Veterinarians Australia, in July 2015; by Dr P Beltz, Animal Standards and Biosecurity Operations in November 2017; by Dr A Lee, Chief Veterinary Officer’s Unit, with the assistance of the Centre for Equine Infectious Diseases, The University of Melbourne, in July 2018.
Published and Authorised by:
Department of Economic Development, Jobs, Transport and Resources
1 Spring Street
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